Have you ever wondered how cooking meat influences its digestibility? It seems like such a simple question – one that should have been answered long ago considering most people eat meat on a daily basis. Alas, it was only recently that Oberli et al from France took to the science to provide an answer. In the first study of its kind, they fed isotopically labelled beef to a small group (n=16) of healthy men and women and followed it through the digestive tract and metabolic pools of the participants.

After consuming a standardized diet (1.4 g/kg protein) for one-week prior to the experimental day, in the morning after an overnight fast, each subject was provided 10 minutes to eat 120 grams (27 g protein) of isotopically labelled minced beef steak (mincing reduces the influence of individual chewing efficiency) that was either  fully-cooked meat [FCM; 90°C (194°F) for 30 minutes] or rare meat [RM; 55°C (131°F) for 5 minutes]. The labeling of the beef protein allowed for the tracking of nitrogen through the body.

The overall procedure must have been very uncomfortable because each participant was required to have a tube enter their nose the previous night and travel all the way down to their ileum while they slept, which is the last portion of the small intestine that connects to the large intestine. This is what allowed the researchers to collect all the unabsorbed effluents for analysis that would have otherwise made it to the colon, thus providing the much needed information on digestibility. The gastric effluents, as well as blood samples, were collected every 30 minutes for 8-hours after meal consumption.

Over the entire 8 hours, there was no significant difference between groups in the total volume of effluents passing through the ileum. However, the nitrogen flux from the meat was significantly greater with FCM than RM, especially because of an increased flux during the first 3-4 hours. Consequently, the amount of nitrogen reaching the large intestine with FCM was greater than with RM, and the overall digestibility of FCM and RM were 90.1 and 94.1 %, respectively. In other words, RM was more digestible than FCM by 4%.

Despite greater digestibility with RM, the overall incorporation of amino acids from the meat into the blood amino acid and protein pools was identical between conditions. Blood amino acids significantly increased in both groups by 1-hour and peaked 2-hours after RM and 3-hours after FCM, with a progressive decline thereafter. Additionally, there were no differences in the amount of protein that was turned to urea and disposed of and net protein utilization was about 70% in both groups.

Red meat has been an important high-quality protein source in the human diet for at least 2-million years, but our control of fire did not emerge in most of Europe, Asia, and Africa until less than 1-million years ago, suggesting that meat was consumed raw for a significant portion of our evolutionary history. Yet, in the modern day, cooking meat is done without a second thought – it’s normal and expected.

This is the first human trial to investigate the impact of cooking on meat protein digestibility and metabolic fate, and the results show that fully cooked meat does have a lower digestibility (90%) than undercooked meat (94%). However, this did not affecting the subsequent postprandial utilization of the consumed protein. The only other human study to look at digestibility of meat in general was performed in five ileostomized patients, with a 94% value reported for meat sautéed for 8 minutes, confirming the current findings.

When meat is cooked, an increase in free radical production and impairment of antioxidant protection leads to oxidative damage that changes the protein structure, resulting in more or less digestibility depending on the cooking parameters and type of protease. Interestingly, an area of controversy with regard to red meat consumption is colon cancer. It is known that proteins that escape digestion are fermented by the gut microbiome and release potentially harmful metabolites such as hydrogen disulfide and ammonia.

The link between colon cancer and red meat may therefore be mediated by the cooking conditions of the meat and the amount of protein that escapes absorption. It is worth mentioning that the digestive value for RM is comparable to that of milk (93%) and greater than that of eggs, soy, pea, and wheat (90-91%). Thus, it would be accurate to state that undercooked meat may actually produce less potentially detrimental protein-fermentation by-products than several common vegan protein sources. Regardless, the difference in the amount of protein that escaped digestion between RM and FCM was about 1 gram with 25-30 grams of consumed protein, and whether this amount has detrimental effects on the colon and our health remains to be determined.

It is also possible that meat cooked somewhere between the RM and FCM used in the current study would be even more digestible. A study in minipigs that cooked meat at various temperatures (60, 75, or 90 °C) for 30 minutes reported that digestibility followed a bell-shaped curve with the greatest being at 75°C. In the current study, RM and FCM were at the far extremes of cooking conditions and future work will need to evaluate the intermediates (medium-rare, medium, etc.).